Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Feb 07 2023 20:27:51 FOUS30 KWBC 072027 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 327 PM EST Tue Feb 07 2023 Day 1 Valid 16Z Tue Feb 07 2023 - 12Z Wed Feb 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS NORTHWESTERN ARKANSAS, SOUTHEASTERN OKLAHOMA, AND PORTIONS OF NORTH TEXAS... ....16Z Update... Very few changes needed for this morning's update. The meteorological conditions described in the previous discussion below remain valid. The biggest change was to trim a few rows of counties off the northeastern side of the Slight and especially Marginal Risk areas in northeastern AR and southeastern MO. Much of the 12Z guidance suggests that the heaviest rainfall will remain west of this region through 12Z Wednesday, and as such the threat for this area is confined to the Day 2 period. The bulk of the heaviest rainfall associated with the risk areas is expected overnight tonight after 00Z, with mostly light shower activity occurring before then. Wegman ....Previous Discussion... A positively-tilted trough will eject from the Four Corners region today, interacting with a jet streak over the Ohio Valley that will strengthen to near 130 kts by late Tuesday/early Wednesday. Deep southerly flow ahead of this system will bring in high levels of tropospheric moisture, characterized by precipitable water values of over 1.25 inches (between 2 and 2.5 standard deviations above the climatological mean). This, combined with strong lift and instability (MUCAPE of up to 1000 J/kg), could result in intense rainfall rates of over 2"/hr at times. Deep layer flow being nearly parallel to expected storm motions may also lead to repeated rounds and training of heavy rainfall locally (with the best odds of this occurring in the Slight Risk area, where HREF 40 km neighborhood probabilities for 3" exceedance are has high as 30-40%). It is also worth noting that NASA SPoRT 0-40 cm relative soil moisture values remain above the 80th or even 90th percentile of climatology locally across portions of eastern TX and southern AR, with 7-day precipitation departures as high as 300-600 percent of normal across much of northern and eastern TX (while soils are drier overall across most of OK and northern AR, as low as the 10-20th percentile). Churchill/Bann Day 2 Valid 12Z Wed Feb 08 2023 - 12Z Thu Feb 09 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF ARKANSAS AND SOUTHERN MISSOURI... ....2030Z Update... Few changes were made for this afternoon's update. The guidance remains in excellent agreement on an area of training convection that will be ongoing at the start of the period across northwestern AR, and extending northeastward into southern MO. Expect multiple rounds of convection through much of the day across this region. While there's great agreement where the heavy rain will occur, there are definite discrepancies on the intensity. The 12Z HREF guidance shows a swath of probabilities over 40% of exceeding 2-year ARIs extending from central into northeastern AR. Soil moisture percentiles from NASA Sport shows soils generally 80-90 percent saturated over the southern half of AR, with quite a bit drier soils in the northwest corner of AR into southern MO. With those dry soils, the biggest changed from the inherited forecast was to trim the northern part of the inherited Slight and especially Marginal Risk areas north of the AR/MO border, as these areas should be able to handle more rainfall before any flooding would occur as compared with areas further south into AR. Soil moisture continues to decline the further north from the AR/MO border one goes. One final cold front pushes all the rainfall to the MS River and points east around 06Z, so the flooding threat from excessive rain will end by then. Wegman ....Previous Discussion... A Slight Risk area has been maintained for the new Day 2 outlook, primarily covering northern Arkansas and southern Missouri (with a small portion in adjoining states). Models continue to show a relatively slow forward speed of the main upper system as it lifts northeastward and takes on a negative-tilt, leading to increased overlap of QPF on Days 1 and 2. The overall setup and forcing remains similar to Day 1, with deep layer southerly flow ahead of the trough increasing tropospheric moisture levels across the MS/OH Valleys, resulting in precipitable water values reaching as high as 1.5-1.8 inches. This increased moisture, along with strong lift and sufficient instability (MUCAPE of 500-1000 J/kg), will result in the potential for heavy rainfall, with areal average rainfall amounts expected to range from 1-3" across the Slight Risk area (and locally up to 3-4 inches in the best overlap areas on Day 1). NASA SPoRT data shows elevated 0-40 cm relative soil moisture values across portions of the Lower MS Valley (as high as the 80-90th percentile across southern AR), while soil moisture values in the Middle MS and OH Valleys are well below normal (as low as the 10th percentile across portions of southern MO/IL/IN and western KY). Churchill/Bann Day 3 Valid 12Z Thu Feb 09 2023 - 12Z Fri Feb 10 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE WESTERN FLORIDA PANHANDLE, SOUTHERN ALABAMA, AND WESTERN GEORGIA... ....2030Z Update... The inherited Marginal Risk area was adjusted primarily on the eastern side to remove much of the eastern FL Panhandle. A squall line of convection is expected to stall out over the area, with the guidance still struggling, but generally agreeing on a more western (slower) stopping point as compared with inherited. Thus, the Marginal Risk area was also expanded a bit towards the northeast to include the Atlanta metro area on the northern end as training convection is likely to develop starting at the Gulf and riding the stalled front northeast into the Atlanta area. Where the heaviest rain falls and whether Atlanta itself is directly impacted remains to be seen as there is some disagreement to that level. Convection also looks to be significantly weaker compared with previous days, so at this point no upgrades beyond the Marginal Risk are expected. Wegman ....Previous Discussion... A Marginal Risk of excessive rainfall has been introduced for the Florida Panhandle and surrounding portions of AL and GA for the new Day 3 outlook, mainly due to a stalling cold front moving into the region. High tropospheric moisture levels are expected ahead of the front, as indicated by GFS progged precipitable water values of 1.5"+. The presence of sufficient instability (MUCAPE of 500-1000 J/kg) will increase the potential for heavy rainfall, with rates possibly exceeding 2"/hr at times. The possibility of repeating rounds of heavy rainfall is also expected, increasing the threat of excessive runoff potential (with ECENS probabilities for 3" exceedance as high as 5% locally). However, antecedent soil conditions in the region are quite dry, with NASA SPoRT 0-40 cm soil moisture values indicated to be as low as the 10-20th percentile. This should significantly limit the areal extent of any flash flooding, with poor drainage/urban areas the most at risk of realized isolated flooding impacts. Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_aMSUdDPXJbL7fDM3ztCcU6R0iSR3Dwpfp7Zj7zoAAp7= Wew033MjTcKdkGurC94HJiAFzftMwFNvCURa0Cd3AHpw1Vo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_aMSUdDPXJbL7fDM3ztCcU6R0iSR3Dwpfp7Zj7zoAAp7= Wew033MjTcKdkGurC94HJiAFzftMwFNvCURa0Cd3U2mRNXA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_aMSUdDPXJbL7fDM3ztCcU6R0iSR3Dwpfp7Zj7zoAAp7= Wew033MjTcKdkGurC94HJiAFzftMwFNvCURa0Cd3aOcNrac$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .